The present invention relates to a frame-supported pellicle for dustproof protection of a photomask used in the patterning works for the manufacture of, for example, fine electronic materials such as semiconductor devices and liquid crystal display panels. More particularly, the invention relates to a frame-supported dustproof pellicle for a photomask, which is almost completely freed from the troubles due to occurrence of dust particles from the pellicle frame per se.
As is known, the photolithographic method is well established in the patterning works for the manufacture of semiconductor devices, such as LSIs, VLSIs and the like, liquid crystal display panels and other fine electronic materials, in which a photoresist layer formed on the surface of a substrate such as a semiconductor silicon wafer is pattern-wise irradiated with ultraviolet light through a pattern-bearing transparency called a photomask followed by development to form a patterned resist layer. In view of the extremely high fineness and precision required in this patterning work, it is very important that the photomask is absolutely dust-free since, when dust particles are deposited on the photomask, the ultraviolet light passing through the photomask is necessarily scattered by the dust particles to give a great adverse influence on the quality of the reproduced pattern such as fidelity to the photomask pattern and contrast of the reproduced images.
It is therefore a usual practice that the photolithographic patterning work is conducted in a clean room under an atmosphere of air freed from any floating dust particles as completely as possible although perfect cleanness can hardly be obtained even in a clean room of the highest class. Accordingly, it is also a usual practice that deposition of dust particles on the photomask is prevented by mounting a frame-supported dustproof pellicle on the photomask. The frame-supported dustproof pellicle mentioned above is an integral device consisting of a frame having parallel end surfaces and made from a rigid material, such as an aluminum alloy, and a thin, highly transparent film of a polymeric resin, which is called a pellicle membrane, spread over and adhesively bonded to one end surface of the pellicle frame in a drumhead-like slack-free fashion. Usually, the other end surface of the pellicle frame, reverse to the end surface to which the pellicle membrane is adhesively bonded, is coated with a pressure-sensitive adhesive so as to ensure reliableness of mounting of the pellicle frame on the photomask.
When a frame-supported pellicle is mounted on the photomask, the dust particles floating in the atmospheric air and falling therefrom are never deposited directly on the photomask but are deposited on the pellicle membrane. Since the light beams used for the pattern-wise exposure to light are focused at the patterned images on the photomask, the dust particles deposited on the pellicle membrane, which is held apart above the photomask by the height of the pellicle frame, have little adverse influences on the quality of the pattern reproduction in the photolithographic patterning work.
Needless to say, all of the products of frame-supported pellicles as manufactured receive very close inspection to ensure complete absence of dust particles deposited on the pellicle membrane before they are encased in a holder case and packaged for transportation and delivery to the users. Nevertheless, it is sometimes the case that dust particles, though very few, are found on the pellicle membrane of the frame-supported pellicle taken out of the unpacked case at the user's site. This trouble is due to nothing other than falling of dust particles held on a certain surface other than the pellicle membrane on to the pellicle membrane as a consequence of vibrations and shocks which the encased pellicle receives during transportation on a truck, railroad car or airplane as well as in handling of the packages. A surface of possibility thought of for such a surface other than the pellicle membrane, which holds the dust particles and from which the dust particles fall on to the pellicle membrane, is the surface of the pellicle frame. In this regard, it would be worthwhile to consider the material of the pellicle frames.
While pellicle frames are made from a rigid material including metallic materials such as aluminum alloys and stainless steels and plastic resins such as polyethylenes, the most conventional material of pellicle frames is an aluminum alloy in view of the high mechanical strengths for its low weight and the surface of the pellicle frame made from an aluminum alloy is usually provided with an oxide film by an anodization treatment to increase the surface hardness and the oxide film is colored in black to prevent stray light. When the surface of the pellicle frame of an aluminum alloy is anodized and colored in black, it has been found that the thus treated surface is microscopically far from smoothness but the anodized oxide film on the surface has a porous structure with ruggedness and is hard but brittle. Accordingly, it is not surprising that dust particles are formed in or from such a rugged and brittle surface of the pellicle frame under a force received in the course of manufacture thereof or by frictionally contacting with the inner surface of the holder case due to unavoidable vibrations and shocks during transportation and handling. For example, it would not be an impossible case that dust particles and particles of the black pigment enclosed in the pores of the porous oxide film on the frame surface are released and scattered into the space to be deposited on the pellicle membrane. The brittle surface layer on the pellicle frame would readily be broken into dust particles when the surface is contacted with the inner surface of the holder case under vibrations and shocks.
A proposal has been made in Japanese Patent Publication 63-777 and Japanese Patent Kokai 64-48062, according to which the inwardly facing surface of a pellicle frame is coated with a pressure-sensitive adhesive or with a sticky coating composition. This method is indeed not ineffective because any dust particles floating within the space surrounded by the frame-supported pellicle and the photomask, when the pellicle is mounted on the photomask, can be captured by hitting at the sticky surface. This method, however, cannot provide a solution for the above described problems in the prior art.